Internet Engineering Task Force (IETF) J. Snijders
Request for Comments: 8195 J. Heasley
Category: Informational NTT
ISSN: 2070-1721 M. Schmidt
i3D.net
June 2017
Use of BGP Large Communities
Abstract
This document presents examples and inspiration for operator
application of BGP Large Communities. Based on operational
experience with BGP Communities, this document suggests logical
categories of BGP Large Communities and demonstrates an orderly
manner of organizing community values within them to achieve typical
goals in routing policy. Any operator can consider using the
concepts presented as the basis for their own BGP Large Communities
repertoire.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8195.
Snijders, et al. Informational [Page 1]

RFC 8195 Use of BGP Large Communities June 20171. Introduction
BGP Large Communities [RFC8092] provide a mechanism to signal opaque
information between and within Autonomous Systems (ASes). In very
much the same way that [RFC1998] provides a concrete real-world
application for BGP Communities [RFC1997], this document presents
examples of how operators might utilize BGP Large Communities to
achieve various goals. This document draws on the experience of
operator communities such as the North American Network Operators'
Group (NANOG) <https://www.nanog.org/> and the Netherlands Network
Operator Group (NLNOG) <https://nlnog.net/>.
2. The Design Overview
BGP Large Communities are composed of three 4-octet fields. The
first is the Global Administrator (GA) field, whose value is the
Autonomous System Number (ASN) of the AS that has defined the meaning
of the remaining two 4-octet fields, known as "Local Data Part 1" and
"Local Data Part 2". This document describes an approach where the
"Local Data Part 1" field contains a function identifier and the
"Local Data Part 2" contains a parameter value. Using the canonical
notation this format can be summarized as "ASN:Function:Parameter".
+----------------------+---------------+
| RFC 8092 | this document |
+----------------------+---------------+
| Global Administrator | ASN |
| Local Data Part 1 | Function |
| Local Data Part 2 | Parameter |
+----------------------+---------------+
Table 1: Field Mapping
The table above shows a mapping table between the fields in BGP Large
Communities [RFC8092] and this document.
In contemporary deployments of both BGP Communities [RFC1997] and BGP
Large Communities [RFC8092], the function of a community can be
divided into two categories:
o Informational Communities
o Action Communities
Snijders, et al. Informational [Page 3]

RFC 8195 Use of BGP Large Communities June 2017
Throughout the document, a topology of four ASes is used to
illustrate the use of communities in the following configuration:
AS 65551
|
^
|
AS 64497
/ \
^ \
/ ^
AS 64498 \
| |
`<->- AS 64499
AS 64497 obtains transit services from (is a customer of) AS 65551, a
4-octet ASN. AS 64497 provides transit services to both AS 64498 and
AS 64499. AS 64498 and AS 64499 maintain a peering relationship in
which they only exchange their customer routes.
The opaque nature of BGP Large Communities allows for rapid
deployment of new features or changes to their routing policy that
perform an action. Operators are encouraged to publicly publish and
maintain documentation on the purpose of each BGP Large Community,
both Informational and Action, that they support or that are visible
in BGP RIBs.
2.1. Informational Communities
Informational Communities are labels for attributes such as the
origin of the route announcement, the nature of the relation with an
External BGP (EBGP) neighbor, or the intended propagation audience.
Informational Communities can also assist in providing valuable
information for day-to-day network operations such as debugging or
capacity planning.
The Global Administrator field is set to the ASN of the network that
tags the routes with the Informational Communities. For example, AS
64497 might add a community with the GA 64497 to a route accepted
from an Internal BGP (IBGP) or EBGP neighbor as a means of signaling
that it was imported in a certain geographical region.
In general, the intended audiences of Informational Communities are
downstream networks and the GA itself, but any AS could benefit from
receiving these communities.
Snijders, et al. Informational [Page 4]

RFC 8195 Use of BGP Large Communities June 20172.2. Action Communities
Action Communities are added as labels to request that a route be
treated in a particular way within an AS. The operator of the AS
defines a routing policy that adjusts path attributes based on the
community. For example, the route's propagation characteristics, the
LOCAL_PREF (local preference), the next hop, or the number of AS_PATH
prepends to be added when it is received or propagated can be
changed.
The Global Administrator field is set to the ASN that has defined the
functionality of that BGP Large Community and is the ASN that is
expected to perform the action. For example, AS 64499 might label a
route with a BGP Large Community containing GA 64497 to request that
AS 64497 perform a predefined action on that route.
In general, the intended audience of Action Communities are transit
providers taking action on behalf of a customer or the GA itself, but
any AS could take action if they choose and any AS could add an
Action Community with the GA of a non-adjacent ASN. However, note
that an Action Community could also be Informational. Its presence
is an indicator that the GA may have performed the action and that an
AS in the AS_PATH requested it.
Operators are recommended to publish the relative order in which
Action Communities (both BGP Communities and BGP Large Communities)
are processed in their routing policy.
3. Examples of Informational Communities3.1. Location
An AS, AS 64497 in these examples, may inform other networks about
the geographical region where AS 64497 imported a route by labeling
it with BGP Large Communities following one of the following schemes
or a combination of them.
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RFC 8195 Use of BGP Large Communities June 20173.1.1. An ISO 3166-1 Numeric Function
AS 64497 could assign a value of 1 to the Function field to designate
the content of the Parameter field as an ISO 3166-1 numeric country
identifier <https://www.iso.org/iso-3166-country-codes.html>.
+---------------------+---------------------------------------------+
| BGP Large Community | Description |
+---------------------+---------------------------------------------+
| 64497:1:528 | Route learned in the Netherlands |
| 64497:1:392 | Route learned in Japan |
| 64497:1:840 | Route learned in the United States of |
| | America |
+---------------------+---------------------------------------------+
Table 2: Informational: ISO 3166-1
The table above shows example documentation for Informational
Communities deployed by AS 64497 to describe the location where a
route was imported using ISO 3166-1 numeric identifiers.
3.1.2. A UN M.49 Region Function
AS 64497 could assign a value of 2 to the Function field to designate
the content of the Parameter field as the M.49 numeric code published
by the United Nations Statistics Division (UNSD)
<https://unstats.un.org/unsd/methodology/m49/> for macro-geographical
(continental) regions, geographical sub-regions, or selected economic
and other groupings.
+---------------------+-------------------------------+
| BGP Large Community | Description |
+---------------------+-------------------------------+
| 64497:2:2 | Route learned in Africa |
| 64497:2:9 | Route learned in Oceania |
| 64497:2:145 | Route learned in Western Asia |
| 64497:2:150 | Route learned in Europe |
+---------------------+-------------------------------+
Table 3: Informational: UNSD Regions
The table above shows example documentation for Informational
Communities deployed by AS 64497 to describe the location where a
route was imported using M.49 numeric codes published by the UNSD.
Snijders, et al. Informational [Page 6]

RFC 8195 Use of BGP Large Communities June 20173.2. Relation Function
An AS, AS 64497 in this example, could assign a value of 3 to the
Function field to designate the content of the Parameter field as a
number indicating whether the route originated inside its own network
or was learned externally, and if learned externally, it might
simultaneously characterize the nature of the relation with that
specific EBGP neighbor.
+---------------------+---------------------------------------+
| BGP Large Community | Description |
+---------------------+---------------------------------------+
| 64497:3:1 | Route originated internally |
| 64497:3:2 | Route learned from a customer |
| 64497:3:3 | Route learned from a peering partner |
| 64497:3:4 | Route learned from a transit provider |
+---------------------+---------------------------------------+
Table 4: Informational: Relation
The table above shows example documentation for Informational
Communities deployed by AS 64497 to describe the relation to the ASN
from which the route was learned.
3.3. Combining Informational Communities
A route may be labeled with multiple Informational Communities. For
example, a route learned in the Netherlands from a customer might be
labeled with communities 64497:1:528, 64497:2:150, and 64497:3:2 at
the same time.
4. Examples of Action Communities4.1. Selective NO_EXPORT
As part of an agreement, often a commercial transit agreement,
between AS 64497 and AS 64498, AS 64497 might expose BGP traffic-
engineering functions to AS 64498. One such BGP traffic-engineering
function could be selective NO_EXPORT, which is the selective
filtering of a route learned from one AS, AS 64498, to certain EBGP
neighbors of the GA, AS 64497.
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RFC 8195 Use of BGP Large Communities June 20174.1.1. ASN-Based Selective NO_EXPORT
AS 64497 could assign a value of 4 to the Function field to designate
the content of the Parameter field as a neighboring ASN to which a
route should not be propagated.
+---------------------+---------------------------------+
| BGP Large Community | Description |
+---------------------+---------------------------------+
| 64497:4:64498 | Do not export route to AS 64498 |
| 64497:4:64499 | Do not export route to AS 64499 |
| 64497:4:65551 | Do not export route to AS 65551 |
+---------------------+---------------------------------+
Table 5: Action: ASN NO_EXPORT
The table above shows example documentation for Action Communities
deployed by AS 64497 to expose a BGP traffic-engineering function
that selectively prevents the propagation of routes to the
neighboring ASN specified in the Parameter field.
4.1.2. Location-Based Selective NO_EXPORT
AS 64497 could assign a value of 5 to the Function field to designate
the content of the Parameter field as an ISO 3166-1 numeric country
identifier within which a labeled route is not propagated to EBGP
neighbors. However, this might not prevent one of those EBGP
neighbors from learning that route in another country and making it
available in the country specified by the BGP Large Community.
+-----------------+-------------------------------------------------+
| BGP Large | Description |
| Community | |
+-----------------+-------------------------------------------------+
| 64497:5:528 | Do not export to EBGP neighbors in the |
| | Netherlands |
| 64497:5:392 | Do not export to EBGP neighbors in Japan |
| 64497:5:840 | Do not export to EBGP neighbors in the United |
| | States of America |
+-----------------+-------------------------------------------------+
Table 6: Action: NO_EXPORT in Region
The table above shows example documentation for Action Communities
deployed by AS 64497 to expose a BGP traffic-engineering function
that selectively prevents the propagation of routes to all EBGP
neighbors in the geographical region specified in the Parameter
field.
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RFC 8195 Use of BGP Large Communities June 20174.2. Selective AS_PATH Prepending
As part of an agreement between AS 64497 and AS 64498, AS 64497 might
expose BGP traffic-engineering functions to AS 64498. One such BGP
traffic-engineering function could be selective prepending of the
AS_PATH with AS 64497 to certain EBGP neighbors of AS 64497.
4.2.1. ASN-Based Selective AS_PATH Prepending
AS 64497 could assign a value of 6 to the Function field to designate
the content of the Parameter field as a neighboring ASN to which
prepending of the AS_PATH with AS 64497 is requested on propagation
of the route. Additional AS_PATH prepending functions might also be
defined to support multiples of prepending, that is, two, three, or
more prepends of AS 64497.
+---------------------+------------------------------------------+
| BGP Large Community | Description |
+---------------------+------------------------------------------+
| 64497:6:64498 | Prepend 64497 once on export to AS 64498 |
| 64497:6:64499 | Prepend 64497 once on export to AS 64499 |
| 64497:6:65551 | Prepend 64497 once on export to AS 65551 |
+---------------------+------------------------------------------+
Table 7: Action: Prepend to ASN
The table above shows example documentation for Action Communities
deployed by AS 64497 to expose a BGP traffic-engineering function
that selectively prepends the AS_PATH with AS 64497 when propagating
the route to the specified EBGP neighbor.
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RFC 8195 Use of BGP Large Communities June 20174.2.2. Location-Based Selective AS_PATH Prepending
AS 64497 could assign a value of 7 to the Function field to designate
the content of the Parameter field as an ISO 3166-1 numeric country
identifier to which the prepending of the AS_PATH with AS 64497 is
requested on propagation of the route to all EBGP neighbors in that
region.
+-----------------+-------------------------------------------------+
| BGP Large | Description |
| Community | |
+-----------------+-------------------------------------------------+
| 64497:7:528 | Prepend once to EBGP neighbors in the |
| | Netherlands |
| 64497:7:392 | Prepend once to EBGP neighbors in Japan |
| 64497:7:840 | Prepend once to EBGP neighbors in the United |
| | States of America |
+-----------------+-------------------------------------------------+
Table 8: Action: Prepend in Region
The table above shows example documentation for Action Communities
deployed by AS 64497 to expose a BGP traffic-engineering function
that selectively prepends the AS_PATH with AS 64497 when propagating
the route to all EBGP neighbors in the geographical region specified
in the Parameter field.
4.3. Manipulation of the LOCAL_PREF Attribute
As part of an agreement between AS 64497 and AS 64498, AS 64497 might
expose BGP traffic-engineering functions to AS 64498. One such BGP
traffic-engineering function might allow AS 64498 to manipulate the
value of the LOCAL_PREF attribute of routes learned from AS 64498
within AS 64497, even though the LOCAL_PREF attribute is
non-transitive and is not propagated to EBGP neighbors.
The LOCAL_PREF value of routes are locally significant within each AS
and are impossible to list in this document. Instead, the typical
LOCAL_PREF values could be classified as a hierarchy, and a BGP Large
Community function could be exposed, allowing an EBGP neighbor to
affect the LOCAL_PREF value within the specified GA. The following
example list defines the classes of routes in the order of descending
LOCAL_PREF value and assigns a function identifier that could be used
in the Function field of a BGP Large Community.
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RFC 8195 Use of BGP Large Communities June 2017
+----------+--------------------------------------------------------+
| Function | Preference Class |
+----------+--------------------------------------------------------+
| 8 | Normal customer route |
| 9 | Backup customer route |
| 10 | Peering route |
| 11 | Upstream transit route |
| 12 | Fallback route, to be installed if no other path is |
| | available |
+----------+--------------------------------------------------------+
Table 9: Action: Preference Function Identifiers
4.3.1. Global Manipulation of LOCAL_PREF
AS 64497 could place one of the previously defined Preference
Function Identifiers in the Function field and set the value 0 in the
Parameter field to designate that the LOCAL_PREF associated with that
function identifier should be applied for that route throughout the
whole AS.
+---------------------+---------------------------------------------+
| BGP Large Community | Description |
+---------------------+---------------------------------------------+
| 64497:9:0 | Assign LOCAL_PREF for a customer backup |
| | route |
| 64497:10:0 | Assign LOCAL_PREF for a peering route |
| 64497:12:0 | Assign LOCAL_PREF for a fallback route |
+---------------------+---------------------------------------------+
Table 10: Action: Global LOCAL_PREF Manipulation
The table above shows example documentation for Action Communities
deployed by AS 64497 to expose a BGP traffic-engineering function
that allows a BGP neighbor to globally manipulate the LOCAL_PREF
attribute for the route within AS 64497.
4.3.2. Region-Based Manipulation of LOCAL_PREF
AS 64497 could place one of the previously defined Preference
Function Identifiers in the Function field and use a UN M.49 numeric
region identifier in the Parameter field to designate the
geographical region within which the non-default LOCAL_PREF
associated with that function identifier should be applied to the
route. The value of the LOCAL_PREF attribute should not deviate from
the default for that route class in any region not specified by one
or more of these Action Communities.
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RFC 8195 Use of BGP Large Communities June 2017
+--------------+----------------------------------------------------+
| BGP Large | Description |
| Community | |
+--------------+----------------------------------------------------+
| 64497:9:3 | Assign the LOCAL_PREF value equivalent to a |
| | customer backup class route on BGP routers in the |
| | North America region |
| 64497:10:5 | Assign the LOCAL_PREF value equivalent to a |
| | peering class route on BGP routers in the South |
| | America region |
| 64497:12:142 | Assign the LOCAL_PREF value equivalent to a |
| | fallback class route on BGP routers in the Asia |
| | region |
+--------------+----------------------------------------------------+
Table 11: Action: Regional LOCAL_PREF Manipulation
The table above shows example documentation for Action Communities
deployed by AS 64497 to expose a BGP traffic-engineering function
that allows a BGP neighbor to selectively manipulate the LOCAL_PREF
attribute within AS 64497 in the geographical region specified in the
Parameter field.
4.3.3. Note of Caution for LOCAL_PREF Functions
The LOCAL_PREF attribute strongly influences the BGP Decision
Process, which in turn affects the scope of route propagation.
Operators should take special care when using Action Communities that
decrease the LOCAL_PREF value, and the degree of preference, to a
value below that of another route class. Some of the unintended BGP
states that might arise as a result of these traffic-engineering
decisions are described as "BGP Wedgies" in [RFC4264].
4.4. Route Server Prefix Distribution Control
Route servers [RFC7947] use BGP to broker network reachability
information among their clients. As not all route server clients may
wish to interconnect with each other, the route server operator will
usually implement a mechanism to allow each client to control the
route server's export routing policy, as described in Section 4.6 of
[RFC7948]. One widely used mechanism is an adaption of "ASN-Based
Selective NO_EXPORT" (Section 4.1.1) that is specific to route
servers.
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RFC 8195 Use of BGP Large Communities June 2017
An example BGP Large Communities policy that enables client-
controlled prefix distribution for a route server operating as AS
64511 is outlined as follows:
+-------------------+-----------------------------------------------+
| BGP Large | Description |
| Community | |
+-------------------+-----------------------------------------------+
| 64511:0:peer-as | Explicitly prevent announcement of route to |
| | peer-as |
| 64511:1:peer-as | Explicitly announce route to peer-as |
| 64511:0:0 | Do not announce route to any peers by default |
| 64511:1:0 | Announce route to all peers by default |
+-------------------+-----------------------------------------------+
Table 12: Action: Route Server Prefix Distribution Control
Multiple BGP Large Community values can be used together to implement
fine-grained route distribution control. For example, route server
client AS 64500 might wish to use a route server for interconnecting
to all other clients except AS 64509. In this case, they would label
all their outbound routes to the route server with 64511:1:0 (to
announce to all clients by default) and 64511:0:64509 (to prevent
announcement to AS 64509).
Alternatively, route server client AS 64501 may have a selective
routing policy and may wish to interconnect with only AS 64505 and AS
64506. This could be implemented by announcing routes labeled with
64511:0:0 (blocking all distribution by default) and 64511:1:64505,
64511:1:64506 to instruct the route server to force announcement to
those two ASNs.
5. Security Considerations
Operators should note the recommendations in Section 11 of "BGP
Operations and Security" [RFC7454] and handle BGP Large Communities
with their ASN in the Global Administrator field similarly.
In particular and in the same respect as BGP Communities [RFC1997],
operators should be cognizant that any Large Community can be carried
in a BGP UPDATE. Operators should recognize that BGP neighbors,
particularly customers and customers of customers, may utilize
communities defined by other BGP neighbors of the operator. They may
wish to send routes with Action Communities and receive routes with
Informational Communities to or from these other neighbors, and it is
beneficial to all to permit this.
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